Photodynamic therapy, also referred to herein as “PDT”, is a process for treating various types of disease such as psoriasis and cancer wherein light irradiation is used to activate a photosensitizing agent which then attacks the target tissue through one or more photochemical reactions, thereby producing a cell-killing, or cytotoxic, effect. It has been discovered that when certain photosensitizer compounds are applied to the human or animal body, they are selectively retained by diseased (e.g., psoriatic or cancerous) tissue while being eliminated by healthy tissue. The diseased tissue containing the photosensitizer can then be exposed to therapeutic light of an appropriate wavelength and at a specific intensity for activation. The light energy, oxygen, and the photosensitizer cause a photochemical reaction which kills the cells in which the photosensitizer resides.
Phthalocyanines represent a known class of photosensitizing agents. The silicon phthalocyanine Pc 4 is currently in early clinical trials for dermatologic conditions, and has demonstrated an excellent safety profile when used topically on skin lesions. The conventional procedure for administration of Pc 4 is application to skin, followed by activation using 675 (+/−5) nm red light after one hour. The interaction of Pc 4, red light, and molecular oxygen in the tissue results in a photodynamic effect, characterized by oxidative stress leading to apoptosis of pathologically relevant cells.
One of the current limitations of topical Pc 4 PDT is the limited cutaneous penetration of Pc 4 formulations. Confocal imaging studies of human skin 1-2 hours after Pc 4 had been applied showed significant absorption in the epidermis, but minimal to no absorption in the papillary dermis. Both the epidermis and the papillary dermis contain immunologic and other cells that it would be preferable to be able to reach in order to effectively treat certain cutaneous pathologies with PDT. Accordingly, a method for providing the delivery of photosensitizer agents such as Pc 4 to deeper tissue levels is needed.